(1) Field of the Invention
The present invention relates to a double-spiral arc tube formed by winding a glass tube into a double spiral, a manufacturing method for the arc tube, and a low-pressure mercury lamp including the arc tube.
(2) Related Art
In the present energy-saving era, a lot of efforts have been made to develop low-pressure mercury lamps. In particular, fluorescent lamps, specifically compact self-ballasted fluorescent lamps that exhibit high luminous efficiency and long life, are calling attentions as light sources alternative to incandescent lamps. Compact self-ballasted fluorescent lamps include arc tubes formed by bending a glass tube and sealing electrodes in the glass tube.
Some of such arc tubes may have a double-spiral structure. As one example, an arc tube with a double-spiral structure may be formed by (a) turning a glass tube at its substantially middle to form a turning part thereof and two spiral parts extending from the turning part to both ends of the glass tube, (b) spirally winding the spiral parts around the same axis, and (c) making end parts of the glass tube substantially parallel with the axis. In such an arc tube, electrodes are inserted and sealed in the end parts of the glass tube that are made substantially parallel with the axis around which the spiral parts are wound (hereafter referred to as the “spiral axis”).
Such a double-spiral arc tube has an advantage over an arc tube formed by connecting a plurality of U-shaped glass tubes. The advantage is that the distance between electrodes within the double-spiral arc tube can be made longer than that in the arc tube formed by connecting a plurality of U-shaped glass tubes, assuming both the arc tubes occupy the same predetermined space. Further, a thin glass tube (with a tube outer diameter of about 9 mm) may be employed for forming such a double-spiral arc tube, and a gap between adjacent spirals of the glass tube in the direction of the spiral axis is set at about 1 mm. By doing so, the number of spirals formed around the spiral axis can be increased without increasing the total length of the arc tube. In this way, arc tubes with the distance between electrodes being long can be obtained, thereby enabling compact self-ballasted fluorescent lamps to produce brightness equivalent to brightness produced by incandescent lamps.
Although having been downsized in recent years, conventional compact self-ballasted fluorescent lamps including double-spiral arc tubes are still larger than incandescent lamps. This fact has been an obstacle to the widespread of such compact self-ballasted fluorescent lamps. As a specific example of problems, when a conventional compact self-ballasted fluorescent lamp with its total length being longer than that of an incandescent lamp is set in an existing lighting apparatus designed for an incandescent lamp, the top part of the lamp may protrude from the lighting apparatus.
In view of that, a first conventional technique proposes a compact self-ballasted fluorescent lamp with a shortened total length, i.e., a lamp including an arc tube with a shortened total length. The arc tube is formed by spirally winding a glass tube with the same pitch from its turning part to its end parts without the end parts being made parallel to the spiral axis, and sealing electrodes in the end parts. A second conventional technique proposes a compact self-ballasted fluorescent lamp in which parallel parts (end parts) of a glass tube are not bent in the direction of the spiral axis, but are bent in the inward direction as disclosed in Japanese Laid-open Patent Application No. H9-17378.
According to the first conventional technique, however, parts of the glass tube extending from the turning part to both ends of the glass tube are spirally wound around the spiral axis, and therefore, gaps between (a) end parts of the glass tube and (b) parts of the glass tube adjacent to the end parts in the direction of the spiral axis are as narrow as about 1 mm. Such narrow gaps fail to provide enough work spaces for sealing the electrodes in the end parts, making the operation of sealing electrodes in the end parts difficult. Further, heating the end parts to seal the electrodes therein causes the adjacent parts of the glass tube to be heated as well, thereby causing these adjacent parts to be deformed, or melted and adhered to the end parts of the glass tube. Such deformed arc tubes are treated as defective products.
According to the second conventional technique, the end parts of the glass tube are bent in the inward direction. In this lamp, therefore, gaps between (a) the end parts of the glass tube and (b) parts of the glass tube adjacent to the end parts are not narrowed, unlike in the case of the first conventional technique. However, these inwardly bent end parts are close to each other, failing to provide enough work spaces for sealing electrodes therein. With such small work spaces, the operation of sealing electrodes in the end parts is difficult.